1. Field of the Invention
The present invention relates to an epitaxial substrate for use in a semiconductor element and particularly to an epitaxial substrate made of a group-III nitride.
2. Description of Related Art
A nitride semiconductor is attracting attention as a semiconductor material for a light-emitting device such as a LED or a LD and for a high-frequency/high-power electronic device such as a HEMT, because the nitride semiconductor has a wide band gap of direct transition type and the breakdown electric field and the saturation electron velocity thereof are high. For example, a HEMT (high electron mobility transistor) device in which a barrier layer made of AlGaN and a channel layer made of GaN are laminated takes advantage of the feature that causes a high-concentration two-dimensional electron gas (2DEG) to occur in a lamination interface (hetero interface) due to the large polarization effect (a spontaneous polarization effect and a piezo polarization effect) specific to a nitride material (for example, see Non-Patent Document 1).
In some cases, a single crystal (a different kind single crystal) having a composition different from that of a group-III nitride, such as SiC, is used as a base substrate for use in a HEMT-device epitaxial substrate. In this case, a buffer layer such as a strained-superlattice layer or a low-temperature growth buffer layer is generally formed as an initially-grown layer on the base substrate. Accordingly, a configuration in which a barrier layer, a channel layer, and a buffer layer are epitaxially formed on a base substrate is the most basic configuration of the HEMT-device substrate including a base substrate made of a different kind single crystal. Additionally, a spacer layer having a thickness of about 1 nm may be sometimes provided between the barrier layer and the channel layer, for the purpose of facilitating a spatial confinement of the two-dimensional electron gas. The spacer layer is made of, for example, AlN. Moreover, a cap layer made of, for example, an n-type GaN layer or a superlattice layer may be sometimes formed on the barrier layer, for the purpose of controlling the energy level at the most superficial surface of the HEMT-device substrate and improving contact characteristics of contact with an electrode.
On the other hand, in the preparation of the above-mentioned nitride device, research and development have been made concerning the use of single crystal silicon for a base substrate for the purpose of reduction of the cost of an epitaxial substrate, and integration with a silicon-based circuit device, and the like (for example, see Patent Documents 1 to 3, and Non-Patent Document 2). In a case where a conductive material such as silicon is selected as the base substrate of the HEMT-device epitaxial substrate, a field plate effect is applied from a back surface of the base substrate, and therefore a HEMT device can be designed for a high breakdown voltage and high-speed switching.
It is already known that, in order that the HEMT-device epitaxial substrate can be structured with a high breakdown voltage, it is effective to increase the total film thickness of the channel layer and the barrier layer and to improve the electrical breakdown strength of both of the layers (for example, see Non-Patent Documents 2 to 5).
However, it is known that forming a nitride film of good quality on a silicon substrate is very difficult as compared with a case of using a sapphire substrate or a SiC substrate, for the following reasons.
Firstly, the values of the lattice constants of silicon and a nitride material are greatly different from each other. This causes a misfit dislocation at an interface between the silicon substrate and a growth film, and facilitates a three-dimensional growth mode at a timing from the nucleus formation to the growth. In other words, this is a factor that hinders the formation of a good nitride epitaxial film having a low dislocation density and a flat surface.
Additionally, the nitride material has a higher thermal expansion coefficient value than that of silicon. Therefore, in the step of lowering, the temperature to the vicinity of the room temperature after a nitride film is epitaxially grown on the silicon substrate at a high temperature, a tensile stress acts in the nitride film. As a result, it is likely that cracking occurs in a film surface and large warping occurs in the substrate.
Moreover, it is also known that trimethylgallium (TMG) that is a material gas of the nitride material for a vapor-phase growth is likely to form a liquid-phase compound with silicon, which is a factor that hinders the epitaxial growth.
In a case where the conventional techniques disclosed in the Patent Documents 1 to 3 and in the Non-Patent Document 1 are adopted, it is possible to cause an epitaxial growth of a GaN film on the silicon substrate. However, the resulting GaN film never has a better crystal quality as compared with a case of using SiC or sapphire for the base substrate. Therefore, preparing an electronic device such as a HEMT using the conventional techniques involves problems of a low electron mobility, a leakage current during the off-time, and a low breakdown voltage.
A method disclosed in the Non-Patent Document 3 exerts a certain effect in improvement in the breakdown voltage of a HEMT device, but it is known that as the film thickness increases, the distance between a substrate and a barrier-layer/channel layer interface increases and consequently the effect of a field plate of a back surface is reduced.
Using a method disclosed in the Non-Patent Document 4 may enable improvement in the breakdown voltage of a HEMT device without largely increasing the film thickness. However, since a portion where a two-dimensional electron gas travels is also formed of a mixed crystal compound, there is a problem that the electron mobility is reduced due to so-called alloy scattering and thus the on-resistance is increased.
Using a method disclosed in the Non-Patent Document 5 may enable increase in the breakdown voltage of a HEMT device while suppressing reduction of the mobility of a two-dimensional electron gas. However, there is a problem that a lamination of GaN and AlGaN causes a band discontinuity and a lattice discontinuity and therefore a concentration of the electric field partially occurs when a high electric field is applied, which results in lowering of the breakdown voltage and increase in the leakage current during the off-time.
In the first place, the disclosure of the Non-Patent Documents 3 to 5 is not for a case of forming a nitride film on a silicon substrate. In order to obtain the breakdown voltage improvement effect as described above even in a case of forming a nitride film, as a prerequisite therefor, a nitride film having a good quality has to be formed on a silicon substrate. However, a method for obtaining compatibility and consistency between ensuring of the quality of a nitride film and improvement in the breakdown voltage properties is neither disclosed nor suggested in any of the Non-Patent Documents 3 to 5.